There's only one problem with this strategy — Nicola Sturgeon already said that scrapping Trident is not a condition for a vote-by-vote deal.

In an interview with Sky News on Thursday the SNP leader said "any confidence-and-supply arrangement would require the non-renewal of Trident". These echoed comments she made on BBC Radio's Good Morning Scotland last month where she said "Trident is a red line" for any formal agreement with Labour — comments that were seized upon as proof that any deal would require negotiations over Britain's nuclear deterrent.

Yet when asked about it directly by The Guardian last month, Sturgeon refused to rule out a deal even with Trident on the table. She said:

"We're unlikely to go into a formal coalition with Labour. It's more likely to be an arrangement where we would support Labour on an issue by issue basis. Under that arrangement there would be many things that we could agree on and that we would support, but we would not vote for Trident."

The key words here are formal coalition/arrangement. A confidence-and-supply deal is where a party commits to vote with a minority government on its budgets and rules out voting against it if a no confidence vote is called by the opposition. It would require the SNP to sign up to Labour's budget plans in advance, making the Trident issue a potential deal-breaker.

However, an issue-by-issue arrangement would mean that the government would have to debate each piece of legislation to get it through parliament.

She has been absolutely clear that while the SNP would not under any circumstances vote for Trident and couldn't agree to a formal confidence-and-supply arrangement with it included, the party would still be comfortable doing a deal with Labour that would allow Ed Miliband to push the existing nuclear deterrent programme forward providing he could get the votes from elsewhere.

Since the Times article, in which the claims were first made by defence secretary Michael Fallon, came out this morning the line of attack has shifted from warning of an existential threat to the UK's defence strategy to worries about the number of submarines that Labour would build (will it be three or four?). This suggests, perhaps, that the Conservatives are aware that their argument may not be watertight.

Only @Conservatives can & will commit to renewing the 4 nuclear submarines required to ensure our national security pic.twitter.com/5jSRirCEdL— CCHQ Press Office (@CCHQPress) April 9, 2015

Yet, if Sturgeon is true to her word, the only way that the Trident programme as a whole would be under threat under a Labour/SNP deal is if the Conservatives themselves vote against it.

So a deal could be done. The open question is whether a Labour government reliant on SNP votes would be able to get a budget through if the latter suspected it would subsequently allocate defence funds towards Trident. If not, then such an administration would have an even shorter shelf-life than most people expect.